The present invention relates to rotation sensors and in particular to multi-turn rotation sensors that can output absolute angular position over a rotational range of greater than 360 degrees.
Rotation sensors including encoders, resolvers and the like, are electromechanical devices providing an electrical output indicating the position of a rotatable shaft. A common type of rotation sensor uses a disk-shaped rotor having an optically readable pattern marked on its surface, the pattern thrilling alternating opaque and transmissive frames. These frames are illuminated from one side by a lamp and light traveling from the lamp through the opaque and transmissive frames of the rotor and then through similar frames in a stationary stator, to be detected by one or more stationary photodetectors. Rotation of the shaft moves the rotor which in turn causes a fluctuation in the light transmitted through the rotor and stator thus producing a signal that may be decoded into a digital indication of shaft movement.
Rotation sensors may be classified as absolute rotation sensors or incremental rotation sensors. Incremental rotation sensors provide only an indication of the change in position of the sensor shaft. In incremental rotation sensors, the rotor normally contains a uniform periodic pattern whose movement past a photodetector creates an index signal indicative of the amount that the shaft has rotated. A separate track may also provide a zero signal for particular angular position. In some cases, one or more photodetectors arranged with an offset of 90 degrees (“quadrature”) provide an indication of the direction of rotation as well as the amount of rotation of the shaft, as is understood in the art.
Absolute rotation sensors, in contrast to incremental rotation sensors, produce a unique value (typically a digital code word) for each rotation sensor position. The rotor of an absolute rotation sensor may carry a series of concentric tracks whose opaque and transmissive segments, examined along a line of radius, reveal a binary or Grey code value indicative of shaft position. Each track provides the value of one bit and is read by a separate photodetector to produce an output digital word.
Often it is desired to have an absolute measure of rotary position over multiple turns (that is, a measurement that spans an angular range of greater than 360 degrees). This can be done using an absolute single-turn rotation sensor by adding an electronic counter that counts each time the value from the rotation sensor “rolls over” from its maximum value to zero. Precise angular position over multiple turns may be done by adding the output from the absolute single-turn rotation sensor to the value of the counter times 360 degrees.
The use of an electronic counter can cause the absolute angular position to be lost in the event of a power failure which causes the electronic counter to reset. In order to create a “non-volatile” multi-turn absolute rotational rotation sensor, a single-turn rotation sensor can be combined with a mechanical counter such as provided by a gear train. In one design, successive gears in the gear train are attached to simple, absolute rotation sensors that provide successive bits in a count value. For example, each gear in the gear train may provide a 2:1 reduction and may incorporate a single bit, absolute rotation sensor. Each rotation sensor then provides a separate binary digit of a count value.
Two alternative approaches use either a battery or electricity developed by Wiegand wires to write to a nonvolatile memory.
The addition of mechanical gear systems, multiple rotation sensors, batteries, or power generation systems to an absolute single-turn rotation sensor to provide multi-turn capability greatly increases the cost, complexity and potential for failure of the resulting rotation sensor.